Defining Adaptive Detection Thresholds

1. An apparatus for use in a wireless communication network, the apparatus comprising: a receiver configured to receive a signal transmitted over a radio channel; and electronic circuitry configured to: generate a correlator output value based on the received signal and a replica of a reference signal, wherein the correlator output value indicates a cross-correlation between the received signal and the replica of the reference signal; adapt a threshold value based on an estimate of a relative amount of noise and interference power in the received signal; compare the correlator output value with the threshold value to detect a presence of a reference signal; and determine a geographical location of the apparatus based on the detected reference signal.

2. The apparatus of claim 1 , wherein the electronic circuitry is further configured to estimate an arrival time of the reference signal.

3. The apparatus of claim 1 , wherein the electronic circuitry is further configured to obtain a reference signal time difference (RSTD) measurement based on a time difference of arrival between reference signals received from two different base stations.

4. The apparatus of claim 1 , wherein the electronic circuitry is further configured to send reference signal time difference (RSTD) measurements to an Evolved-Serving Mobile Location Center (E-SMLC) using a Long-Term Evolution (LTE) Positioning Protocol (LPP) protocol.

5. The apparatus of claim 1 , wherein the detected reference signal comprises a positioning reference signal (PRS).

6. The apparatus of claim 1 , wherein the electronic circuitry is configured to perform a cross correlation in a time domain and wherein the electronic circuitry is further configured to use the cross correlation to derive the correlator output value.

7. The apparatus of claim 1 , wherein the electronic circuitry is configured to perform a cross correlation in a frequency domain and wherein the electronic circuitry is further configured to use the cross correlation to derive the correlator output value.

8. The apparatus of claim 7 , wherein the electronic circuitry is further configured to: use a delayed output signal in the frequency domain and the complex conjugated replica of the reference signal in the frequency domain to provide a frequency domain product; perform complex addition of the frequency domain product to provide a summed frequency domain product; convert the summed frequency domain product to a time domain summed product; and use the time domain summed product to provide a correlator output.

9. The apparatus of claim 1 , wherein the electronic circuitry is further configured to adapt the threshold value to the estimate of the relative amount of noise and interference power in the received signal by performing an interpolation between a pure noise threshold and a pure interference threshold.

10. The apparatus of claim 9 , wherein the electronic circuitry is further configured to perform the interpolation as a linear interpolation or as an interpolation in the logarithmic domain.

11. The apparatus of claim 9 , wherein the pure noise threshold is dependent upon an inverse of the cumulative distribution function (2M) at a 1−P fa level, where M is an integer number of OFDM segments of the reference signal and where P fa is a false alarm value.

12. The apparatus of claim 11 , wherein the pure interference threshold is dependent upon an expression λ * = 1 2 ⁢ M ⁢ F QPSK - 1 ⁡ ( 1 - P fa ; M , N 1 , N 2 , N c ) , where M is an integer number of OFDM segments of the reference signal, each segment consisting of N c number of OFDM symbols, wherein N c is an integer accumulation length of the reference signal, wherein N 1 is the total number of PRS symbols from subcarriers containing one PRS symbol per subframe, wherein N 2 denote the total number of PRS symbols from subcarriers containing two PRS symbol per subframe, wherein F QPSK (1−P fa ; M, N 1 , N 2 , N c ) is a cumulative density function dependent upon (1−P fa ), and wherein P fa is a false alarm value.

13. The apparatus of claim 1 , the electronic circuitry is further configured to adapt the threshold value to the estimate of the relative amount of noise and interference power in the received signal by performing a noise-weighted interpolation between a pure noise threshold and a pure interference threshold using an estimate of a noise weight factor.

14. The apparatus of claim 13 , wherein the electronic circuitry is further configured to determine the estimate of the noise weight factor based on an estimate of a normalized fourth moment of the channel-propagated signal from the radio channel.

15. The apparatus of claim 1 , wherein the electronic circuitry is further configured to adapt the threshold value to the estimate of the relative amount of noise and interference power in the received signal by scaling and convolving a quantized Gaussian distribution with a scaled binomial distribution.

16. A method of operating a wireless communication device, the method comprising: generating a correlator output value based on the received signal and a replica of a reference signal, wherein the correlator output value indicates a cross-correlation between the received signal and the replica of the reference signal; adapting a threshold value based on an estimate of a relative amount of noise and interference power in the received signal; comparing the correlator output value with the threshold value to detect a presence of a reference signal; and determine a geographical location of the apparatus based on the detected reference signal.

17. The method of claim 16 , further comprising obtaining a reference signal time difference (RSTD) measurement based on a time difference of arrival between reference signals received from two different base stations.

18. The method of claim 16 , further comprising sending reference signal time difference (RSTD) measurements to an Evolved-Serving Mobile Location Center (E-SMLC) using a Long-Term Evolution (LTE) Positioning Protocol (LPP) protocol.

19. The method of claim 16 , wherein the detected reference signal comprises a positioning reference signal (PRS).

20. The method of claim 16 , further comprising estimating an arrival time of the reference signal.

21. The method of claim 16 , further comprising performing a cross correlation in a time domain and using the cross correlation to derive the correlator output value.

22. The method of claim 16 , further comprising performing a cross correlation in a frequency domain and using the cross correlation to derive the correlator output value.

23. The method of claim 22 , further comprising: using the delayed output signal in the frequency domain and the complex conjugated replica of the reference signal in the frequency domain to provide a frequency domain product; performing complex addition of the frequency domain product to provide a summed frequency domain product; converting the summed frequency domain product to a time domain summed product; and, using the time domain summed product to provide a correlator output.

24. The method of claim 16 , further comprising adapting the threshold value to the estimate of the relative amount of noise and interference power in the received signal by performing an interpolation between a pure noise threshold and a pure interference threshold.

25. The method of claim 24 , further comprising performing the interpolation as a linear interpolation or as an interpolation in the logarithmic domain.

26. The method of claim 24 , further comprising adapting the threshold value to the estimate of the relative amount of noise and interference power in the received signal by performing a noise-weighted interpolation between a pure noise threshold and a pure interference threshold using an estimate of a noise weight factor.

27. The method of claim 26 , further comprising determining the estimate of the noise weight factor based on an estimate of a normalized fourth moment of the channel-propagated signal from the radio channel.

28. The method of claim 27 , wherein the pure noise threshold is dependent upon an inverse of the cumulative distribution function (2M) at a 1−P fa level, where M is an integer number of OFDM segments of the reference signal, and where P fa is a false alarm value.

29. The method of claim 16 , wherein the pure interference threshold is dependent upon an expression λ * = 1 2 ⁢ M ⁢ F QPSK - 1 ⁡ ( 1 - P fa ; M , N 1 , N 2 , N c ) , where M is an integer number of OFDM segments of the reference signal, each segment consisting of N c number of OFDM symbols, wherein N c is an integer accumulation length of the reference signal, wherein N 1 is the total number of PRS symbols from subcarriers containing one PRS symbol per subframe, wherein N 2 denote the total number of PRS symbols from subcarriers containing two PRS symbol per subframe, wherein F QPSK (1−P fa ; M, N 1 , N 2 , N c ) is a cumulative density function dependent upon (1−P fa ), and wherein P fa is a false alarm value.

30. The method of claim 16 , further comprising adapting the threshold value to the estimate of the relative amount of noise and interference power in the received signal by scaling and convolving a quantized Gaussian distribution with a scaled binomial distribution.